PATCH /milestones/{id}/backlog with "add" properties

-> Subtleties:
* There is no remove here as there is no way to remove something from backlog.
* Add will only check if the given artifact is from one of the possible tracker
  for submilestones (ie. user story if sprints).
  This could lead to inconsistencies if wrongly used (but nothing worst than
  modifying tracker artifact links by hand)

Change-Id: Ib0b2a7b06938f524fa677efc71f37ff357d9cbe9
Merge commit 'refs/changes/43/3343/3' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  PATCH /milestones/{id}/backlog with "add" properties

Change-Id: Ib2efe4ee1ba5fed8fff4425280a226b989f41724
This is Tuleap 7.7.99.74

Change-Id: I5adf39d933bc0044350078135098fdbcfd3e26fa